Throughout the specification, including in the claims, the expressions "earth" and "earth formation" are used in a broad sense to denote any nonhomogeneous, dispersive medium having complex permittivity. Important examples of such media include subterranean formations such as soil, hardpan, and bedrock, and bodies of water (such as the ocean) above such subterranean formations.
Various systems have been proposed for transmitting electromagnetic signals into the earth subsurface from airborne, subsurface, and surface equipment, and for receiving and processing electromagnetic radiation that has propagated through the subsurface.
For example, U.S. Pat. No. 2,077,707, issued Apr. 20, 1937, discloses transmission of electromagnetic radiation into the earth by passing current through a coiled wire antenna, or by causing current to flow between two electrodes disposed in the earth. U.S. Pat. No. 2,077,707 teaches that after the radiation has propagated through the earth, it should be detected within one or more narrow frequency bandwidths.
U.S. Pat. No. 1,365,579, issued Jan. 11, 1921, also discloses an apparatus capable of transmitting electromagnetic radiation into the earth. U.S. Pat. No. 1,365,579 suggests the technique of impressing high frequency oscillations on bent wires embedded in the earth (wires C2 and C3 shown in FIGS. 2 and 3). Each bent wire has a pair of separated, horizontally oriented end portions.
However, it had not been known until the present invention how efficiently to transmit a short pulse of electromagnetic radiation (a broadband signal including high frequency components) into the earth, and then detect the resulting return signal (electromagnetic radiation that has propagated through the earth) and process low frequency components of the return signal to image the earth. In particular, it had not been known efficiently to transmit such broadband radiation from an efficiently designed, small, transmitting capacitor in contact with the earth, where the capacitor has small size in comparison with the wavenumber (in the earth) of each frequency component of the radiation transmitted into the earth.
The prior art has described the fields that would result in theory from such a small transmitting capacitor, in the idealized case that the capacitor is disposed in a uniform dielectric medium and driven by an ideal delta-function generator. For example, the articles by King, et al., entitled "The Electrically Short Antenna as a Probe for Measuring Free Electron Densities and Collision Frequencies in an Ionized Region," Journal of Research of the National Bureau of Standards, Vol. 65, No. 4, July-August 1961, pp. 371-384, and "Half-Wave Cylindrical Antenna in a Dissipative Medium: Current and Impedance," Journal of Research of the National Bureau of Standards, Vol. 64D, No. 4, July-August 1960, pp. 365-380, discuss the fields resulting from a cylindrical electrical probe, having length 2h and radius a, immersed in a conducting dielectric medium. King, et al. (1961) assume that bh&lt;1 and ah&lt;1, where "b" is the real part of the wavenumber (of radiation in the medium) and "a" is the imaginary part of the wavenumber (of radiation in the medium).
However, this prior art teaching is insufficiently sophisticated to predict the fields that would result from a transmitting capacitive probe in the very complicated case that the probe is disposed in a nonhomogeneous dispersive medium such as a subterranean formation. The prior art neither discloses nor suggests the technique of efficiently transmitting broadband electromagnetic radiation (including high frequency components) into the earth using a transmitting capacitor. Nor does the prior art suggest how to maximize the ratio of "power coupled into the earth to power coupled to the air" from a transmitting capacitor disposed on the earth surface, by appropriately selecting the size, shape, and impedance of the capacitor, and the frequency content of the transmitted signal. Nor does the prior art disclose or suggest the step of detecting a return signal that results from propagation of a broadband pulse of electromagnetic radiation from a capacitor through the earth, or the step of processing low frequency components of such a return pulse to image the earth.